Cross-Sectional TEM Study of Phase Separation in Reaction of Ni-Ta Films with GaAs
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A. Lahava), M. Eizenberg and Y. Komem Department of Materials Engineering, Technion, Haifa, 32000, Israel ABSTRACT The reaction between Ni60Ta4O amorphous alloy and (001) GaAs was studied by cross-sectional transmission electron microscopy, Auger spectroscopy, and x-ray diffraction. At 400%C formation of Ni GaAs at the interface with GaAs was observed. After heat treatment at 600°1 in vacuum a layered structure of TaAs/NiGa/GaAs has been formed. The NiGa layer has epitaxial relations to the GaAs substrate. The vertical phase separation can be explained by opposite diffusion directions of nickel and arsenic atoms. INTRODUCTION The phase separation has been studied extensively in alloy contacts to silicon due to its' potential application in formation of shallow contacts to VLSI devices [1,21. The alloy films were composed of two codeposited metals, one of which (usually a near-noble metal, e.g., Pt, Pd, Ni) forms silicide at low annealing temperature, while the other one (a refractory metal, e.g., W, Mo, Ta) reacts with Si at relatively high temperature. The reaction starts by dealloying of the film (increase of refractory metal content) accompanied by formation of silicide of the near-noble metal at the interface with Si. The further progress of the reaction results in formation of a silicide of the refractory metal in the upper layer. The impetus for this work was to determine whether the reaction between alloy films and GaAs will yield similar phase separation. Ni and Ta were chosen as representatives of near-noble and refractory metals, respectively. The alloy film composition was chosen as Ni 60Ta40, since in the as-deposited state this alloy is characterized by amorphous structure with high crystallization temperature [3] and can be potentially used as a diffusion barrier. In this paper the results of cross-sectional transmission electron microscopy (TEM), Auger electron spectroscopy (AES) and x-ray diffraction (XRD) measurements of the reacted contacts are reported. EXPERIMENTAL Alloy films 250 nm in thickness were deposited on (001) GaAs substrates by coevaporation of Ta from an e-gun and Ni from a resistively heated tungsten boat. The samples were annealed for 1 hour in vacuum of 3x10 -7 torr in the temperature range of 100 - 650°C. The TEM study was performed at 120keV on a Jeol 100 CX scanning transmission electron microscope, equipped with energy dispersive x-ray spectroscopy (EDS) which allows for in-situ compositional analysis of the thin film area of about 5 nm2. The vertical cross-section TEM samples were prepared by ion milling [4]. a) Present address: AT&T Bell Labs, 600 Mountain Ave., Murray Hill, NJ 07974 Mat. Res. Soc. Symp. Proc. Vol. 54.
1986 Materials Research Society
356
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